I think the answer that we are looking here should be more inline with how minus operator works in compareTo.

Assuming id to be an Integer, this.id - emp.id might give us erroneous results because of overflow, once integer crosses the max range in Java. Since, compareTo method only cares about the result being +ve or -ve, this might lead to absurd behaviour in the program.

One of the ways to achieve this neatly will be to use Integer.compareTo().